1. Field of the Invention
This invention relates to an analog-to-digital converter and, more particularly, to an improvement of a dual-slope integrating analog-to-digital converter.
2. Description of the Prior Art
FIG. 1 shows a schematic block diagram of a typical conventional dual-slope integrating analog-to-digital converter (referred to as an A-D converter). FIG. 2 shows a waveform for illustrating the operation of the A-D converter. In the conventional A-D converter, an unknown analog voltage V.sub.IN is applied through a switch circuit 1 to an integrator circuit 5 comprised of a resistor 2, a capacitor 3, and an operational amplifier 4 and integrated during a given time interval T1. Then the switch 1 is operated to cause the integrator 5 to integrate a reference voltage -V.sub.R with the opposite polarity to the analog input voltage V.sub.IN. A counter circuit 7 counts a time interval T2 from the end of the time interval T1 to a point of time at which the output of the integrator 5 reaches a threshold voltage (comparison reference voltage) Vc applied to the inverting input of a comparator 6 whose noninverting input is connected to the output of the integrator 5. A control circuit 8 in FIG. 1 controls the switching operation of the switch circuit 1 and responds to the output of the comparator 6 to transfer a clock pulse Cp to the counter circuit 7. In general, ground voltage is applied to the noninverting input terminal of the integrator 5, and the comparison reference voltage Vc applied to the comparator 6 is ground voltage, as shown in FIG. 1.
In the abovementioned A-D converter, the unknown analog voltage V.sub.IN and the digital amount T2 obtained after the A-D conversion are related by EQU V.sub.IN =T2/T1.times.V.sub.R ( 1)
Equation (1) shows that, if the time interval T1 and the reference voltage -V.sub.R are previously known and the time interval T2 is measured by using the clock pulse Cp, the digital value corresponding to the unknown analog voltage V.sub.IN is obtained.
The conventional A-D converter system has some disadvantages as listed below.
1. The reference voltage -V.sub.R and the unknown analog voltage V.sub.IN must be opposite in polarity. This condition imposes a great restriction on designing of the A-D converter. Particularly, this makes it impossible to realize a ratiometric conversion having an extensive application. PA0 2. Since ground potential is coupled with the noninverting input terminal of the integrator 5, the integrator 5 needs two drive power sources (positive and negative). The same condition is applicable for the comparator 6. The need of two power sources for the A-D converter increases cost and is disadvantageous in many system applications. Particularly, when it is used as an A-D converter in a data collection system using a microcomputer, recently finding a wide application, and is integrated on a semiconductor substrate, the conventional A-D converter is impracticable because the microcomputer system is generally driven by only a single power source. PA0 3. The range where the conventional A-D converter can convert the unknown analog input voltage V.sub.IN from its analog form to digital form extends generally from ground to the reference voltage .vertline.V.sub.R .vertline.. As seen from the integration waveform in FIG. 2, when the analog input voltage V.sub.IN is extremely close to ground voltage, the integration waveform remains close by the threshold voltage level Vc of the comparator 6 during the period T1. Therefore, the input condition of the comparator 6 becomes very unstable and thus the A-D converter is liable to malfunction. As a result, a high precision A-D conversion cannot be expected. Additionally, when the analog input voltage V.sub.IN is slightly lower than ground voltage, the A-D converter cannot carry out an A-D conversion of such an analog voltage. PA0 4. In the conventional A-D converter, due to an offset Vos in the input voltage, which is inherent to the operational amplifier 4 in the integrator 5, a high precision A-D conversion is impossible. In order to obtain high precision data, therefore, additional circuit means must be used so as to eliminate the error resulting from the offset voltage. One approach is to manually adjust the offset voltage to zero for each A-D conversion. Another approach is that the offset voltage is stored in a relatively large capacitor and the stored voltage is superimposed on the voltage to be integrated so as to effectively eliminate the influence of the offset voltage. However, when the A-D converter is used in a data collecting system as mentioned above, manual operation is unsuitable for system maintenance. Further, the use of the capacitor increases the number of parts and thus its manufacturing cost becomes high.